1. Field of the Invention
The present invention relates generally to an ultrasonic length measuring apparatus and method for measuring the length by counting the propagation time of ultrasonic waves and, more particularly, to an ultrasonic length measuring apparatus and method for coordinate inputting, as character data or graphic data, tracks created by hand-writing characters or figures on a sheet of paper with a dedicated pen.
2. Description of the Related Arts
Mice, trackballs and magnetic-type or pressure-sensitive-type tablets etc. are known as coordinate inputting apparatuses (pointing devices) for inputting the coordinate information to the computer. Among these, the magnetic-type or pressure-sensitive-type tablets are excellent in terms of usability and precision. This is a pointing apparatus with which, when a magnetic-type or pressure-sensitive-type sensor plane is traced with a pen-type mover, the coordinate positions on the sensor plane traced are detected and their position information is output as electric signals. The magnetic-type or pressure-sensitive-type tablet has an excellent characteristic as above but the area for writing operation is limited within the sensor plane attached to it. If the area for writing is extended, a larger apparatus is necessary as an apparatus constituting the sensor plane and, in this case, the cost of the sensor plane unit increases as well as a large space for accommodating the sensor plane unit is necessary.
Conventionally, length measuring apparatuses using ultrasonic wave employed in coordinate inputting apparatuses obtain a length by receiving an ultrasonic wave transmitted by a transmitter and measuring the propagation time periods of the wave. Because the length to be measured is obtained by multiplying the propagation time period by the velocity of the ultrasonic wave, the length is calculated by measuring the time between the time the wave is transmitted and the time the wave arrives at the receiver as the propagation time period. A proper threshold is set for the received waveform of the ultrasonic wave that has been converted into an electric signal after being received by the receiver and the time the signal exceeds the threshold is determined as the time the wave arrives at the receiver. In order to obtain a length accurately, it is necessary to always detect the arriving timing with a same waveform as the received waveform of the ultrasonic wave from the receiver.
Because an ultrasonic wave becomes attenuated as the length the wave has propagated becomes longer, the form of an ultrasonic wave received at the receiver varies depending on the length. FIG. 1 shows a form of an ultrasonic wave received. In FIG. 1, the waveform is the form of an ultrasonic wave 1100 when the length is short and the wave is attenuated to have the form as an ultrasonic wave 1102 represented by the dotted line when the length is long. The ultrasonic wave actually arrives is not only attenuated. An ultrasonic wave sensor made from an ordinary piezoelectric ceramic used as the receiver is very sensitive but the form of the ultrasonic wave received slowly becomes larger as shown in FIG. 1 because of the high Q-value. Therefore, if the arriving time is obtained by simply obtaining the timing when the waveform crosses the threshold Vth1, the timing is a detected timing 1104 when the length is short but the timing is a detected timing 1106 when the length is long and the amplitude of the detected waveform has been attenuated because the threshold Vth1 and the wave crossing it are different. As a result, an error corresponding to one wavelength is produced. For example, when utilizing an ultrasonic wave at the frequency of 40 KHz, dislocation of one wavelength causes an error of as much as 8 mm because one wavelength corresponds to approximately 8 mm. As the approaches for reducing this influence of amplitude of ultrasonic waves, a method utilizing the envelope of the form of the ultrasonic wave received (for example, Japanese Patent Application Laid-open (kokai) No. Hei5-215850) and a method for obtaining an assumed zero-cross point utilizing a plurality of each apex of wave of the form of an ultrasonic wave (for example, Japanese Patent Application Laid-open (kokai) No. Hei8-254454) etc. have been proposed. In the case of the method utilizing the envelope, the wave crossing the threshold is not mistaken like in the case utilizing directly the waveform of an ultrasonic wave but the temporal resolution can not made high. In the method in which an assumed zero-cross point is obtained, a complicated, i.e., expensive signal processing circuit is necessary in which a peak-detector is conducted at a plurality of levels for the received waveform and the assumed zero-cross point is obtained by calculation. Therefore, a method in which the signal processing circuit is simple and low-cost but a high temporal resolution can be obtained is being sought.
The uses which need such a high resolution include, for example, a case where a coordinate of a position is measured from two lengths measured by receiving an ultrasonic wave from an ultrasonic wave transmission unit attached to a pen etc. using two ultrasonic wave sensors fixedly mounted and the movement of the pen accompanying the writing down of characters on a sheet of paper is input into an information processing apparatus, and cases where the mounting spacing of ultrasonic wave sensors is much smaller compared to the length to be measured.
With the prevalence of personal computers in recent years, inputting electronic texts using a keyboard is getting more frequent than hand-writing characters on a sheet of paper using pencil etc. However, handwriting characters etc. actually on a sheet of paper is easier than inputting with a keyboard when taking a brief note. Therefore, when attending a meeting, it is still common that each person present brings a scratchpad and hand-writes into it using a pencil. However, considering that the note taken has to be handed out to others or managed, it is preferable that the note taken is electronized. Especially, because the process necessary for the electronization if the hand-written characters are read out by an image scanner and it is inconvenient to carry if a touch-panel or a tablet is employed, it is desirable to be able to input the hand-written characters easily as electronized data. From the above-described facts, conventionally, a coordinate input apparatus is known in which a pair of ultrasonic wave receivers are arranged on a sheet of paper and, by receiving an ultrasonic wave transmitted from a pen by these ultrasonic wave receivers and from triangulation using the propagation time period of the ultrasonic wave, the coordinates are input. For example, Japanese Patent Application Laid-open (kokai) No. Hei8-36462 and Japanese Patent Application Laid-open (kokai) No. 2000-298547 disclose a technique in which two ultrasonic wave receivers constituting a receiver are arranged on a desk or support base and the length between a pen and the ultrasonic wave receiver is continuously measured from the propagation time period necessary for an ultrasonic wave to propagate to there ultrasonic wave receivers and, then, the coordinates of the pen against the desk is obtained using triangulation. In this case, an infrared ray and ultrasonic wave receiver is provided on the desk and an infrared ray from the pen is received by the infrared ray and ultrasonic wave receiver to make the temporal synchronization of the time when an ultrasonic wave was generated from the pen. However, when such a conventional technique employing triangulation is used, an area being a blind spot is created in a portion of the sheet of paper and the problems that inputting coordinates becomes impossible or the resolution becomes degraded consequently even if the input is possible occur. In addition, problems of larger size of the apparatus and increased cost occur.